The mitosome has been detected only inanaerobic ormicroaerophilic eukaryotes which do not have fully developedmitochondria, and hence do not have the capability of gaining energy from mitochondrialoxidative phosphorylation.[2] The functions of mitosomes, while varied, have not yet been well characterized,[2] but they may be associated with sulfatemetabolism andbiosynthesis ofphospholipids andFe–S clusters.[2][6][8][9] Mitosomes, like other MROs, likely evolved from mitochondria,[3][10] based on similarities in structure, function, and biochemical signaling pathways,[3][4][5][6][10] and may have convergently evolved across eukaryote lineages.[2][9]
Mitosomes are membrane-bound organelles closely related to mitochondria in structure, though functional overlap is limited.[2][3] Unlike mitochondria, mitosomes do not havegenes within them; instead, the genes for mitosomal components are contained in thenuclear genome.[3] An early report suggested the presence ofDNA in this organelle,[11] but subsequent research has shown this not to be the case.[12] Manyproteins within mitosomes (e.g., inGiardia intestinalis) have poorly resolved or unexplored functions which are likely related to metabolism and protein transport.[13] Unlike mitochondria, mitosomes appear to lackelectron transport chains, N-terminal targeting sequences, and the ability to fuse with each other.[9]
Current knowledge indicates mitosomes probably play a role inFe–S cluster assembly, since they do not display any of the proteins involved in other major mitochondrial functions (oxidative phosphorylation viaaerobic respiration,haem biosynthesis) while they do displayproteins required for Fe–S cluster biosynthesis (likefrataxin,cysteine desulfurase,Isu1 and a mitochondrialHsp70).[2][6][9] Additionally, modified mitosomes in the intracellular parasitic protistParamikrocytos canceri may biosynthesize phospholipids and support glycolyticATP production, based on genomic and transcriptomic analysis.[2] Mitosomes may also facilitate metabolic activation ofsulfates in some eukaryotes, based on analyses of enzymes from mitosomes inEntamoeba histolytica andMastigamoeba balamuthi.[8][14] Recent work indicates that mitosomes participate in the transformation ofEntamoeba histolyticatrophozoites into cysts, thereby playing a key role in the pathogenic life cycle of this organism,[14] though the role of mitosomes in pathogenicity is less clear for many other parasitic eukaryotes.[9]
In the most widely accepted view, mitosomes are ultimately derived frommitochondria, and commonalities between the protein transport and signaling networks of mitochondria,hydrogenosomes (a related class of MROs), and mitosomes have been interpreted as relics of their commonendosymbiotic origin.[9][10] Like mitochondria, they have a double membrane and most proteins are delivered to them by atargeting sequence of amino acids.[3][5][6] The targeting sequence is similar to that used for mitochondria and true mitochondrial presequences will deliver proteins to mitosomes.[3] A number of proteins associated with mitosomes have been shown to be closely related to those of mitochondria[4] andhydrogenosomes.[15]
Mitosomes appear to have degeneratively evolved from mitochondria multiple times across eukaryote lineages,[2] and their "mosaic" biochemistry inEntamoeba histolytica may reflect a composite ancestry involving both eukaryotes andproteobacteria.[8] It has been proposed that MROs such as mitosomes evolved in anoxic marine environments which predominated during theProterozoic, thus explaining their anaerobic metabolic functionality.[16]
^Tovar J, León-Avila G, Sánchez LB, Sutak R, Tachezy J, van der Giezen M, et al. (November 2003). "Mitochondrial remnant organelles of Giardia function in iron-sulphur protein maturation".Nature.426 (6963):172–176.Bibcode:2003Natur.426..172T.doi:10.1038/nature01945.PMID14614504.S2CID4402808.
^abcDolezal P, Makki A, Dyall SD (2019). "Protein Import into Hydrogenosomes and Mitosomes". In Tachezy J (ed.).Hydrogenosomes and Mitosomes: Mitochondria of Anaerobic Eukaryotes. Microbiology Monographs. Vol. 9. Cham: Springer International Publishing. pp. 31–84.doi:10.1007/978-3-030-17941-0_3.ISBN978-3-030-17941-0.
^Zimorski V, Martin WF (2019). "The Evolution of Oxygen-Independent Energy Metabolism in Eukaryotes with Hydrogenosomes and Mitosomes". In Tachezy J (ed.).Hydrogenosomes and Mitosomes: Mitochondria of Anaerobic Eukaryotes. Microbiology Monographs. Vol. 9. Cham: Springer International Publishing. pp. 7–29.doi:10.1007/978-3-030-17941-0_2.ISBN978-3-030-17940-3.S2CID202026532.
